1. Field of the Invention
The present invention relates to a display unit, particularly to a driving circuit of a flat panel display unit.
2. Description of Related Art
A portable information terminal unit represented by a cellphone (hereafter referred to as cellphone) is generalized. A display unit for displaying the information obtained through communication is set to a presently generalized portable terminal. A display unit to be mounted on a portable terminal generally uses a display unit using an LCD (Liquid Crystal Display) (hereafter referred to as liquid-crystal display). The liquid-crystal display includes an LCD panel for displaying characters and images and an LCD driving circuit and the LCD panel is driven by the LCD driving circuit.
The LCD panel has a display contrast characteristic specific for the LCD panel. In general, an LCD panel manufactured by a panel maker is shipped to a set maker and built in a liquid-crystal display. To keep all liquid-crystal displays manufactured by the set maker at an optimum display quality, it is necessary to fine-adjust the display contrast for each LCD panel when they are shipped to the set maker (for example, VCOM adjustment and LCD driving-voltage setting-value adjustment) and decide an optimum control register value. A conventional LCD driving circuit includes an EEPROM for storing the control register value (for example, refer to Patent Document 1). The conventional display unit keeps a control register value decided when the display unit is shipped from a plant by writing the value in an EEPROM. Therefore, a display unit after shipped drives an LCD panel at a proper display quality when an EEPROM driving circuit reads a register value.
FIG. 1 is a block diagram showing a configuration of a conventional liquid-crystal display. As shown in FIG. 1, the conventional liquid-crystal display is constituted by including an LCD driving circuit 101, LCD panel 102, and CPU 103. The LCD driving circuit 101 includes an LCD control driver and an EEPROM. The LCD driving circuit 101 is connected to the LCD panel 102 and CPU 103 and generates a control signal for driving the LCD panel 102 by responding to a display instruction output from the CPU 103. The LCD panel 102 has a specific display contrast characteristic for each panel.
An LCD control driver 104 set to the LCD driving circuit 101 is a control circuit for performing LCD display operation control (function for displaying a character or image) by responding to a display instruction sent from the CPU 103. Moreover, the LCD control driver performs the display quality control (adjustment of facing-electrode signal VCOM and adjustment of LCD driving-voltage set value) of the LCD panel 102 in accordance with a set value read from an EEPROM. The EEPROM is an information memory for storing the information (register value) on the display quality of the LCD panel 102. In the case of a conventional liquid-crystal display, a register value output from an EEPROM is supplied to an LCD control driver and thereby, the display quality of the LCD panel 102 is kept in a proper state.
As shown in FIG. 1, the conventional liquid-crystal display does not include an area for storing the backup data of the register value. Therefore, when the EEPROM malfunctions or data in the EEPROM is lost, an abnormal display state may appear. In this case, a technique is known which prevents unexpected data from being erroneously set by setting the register value to a default value (for example, refer to Patent Document 2). In the case of the technique disclosed in Patent Document 2, each register value is set to a default value at the time of an EEPROM error and default setting is kept until a register is reset from a CPU. When the default setting is setting which cannot be displayed such as stop of a clock signal, display disappears until the register is reset from the CPU. Moreover, address and data common to every liquid-crystal display which are not necessary in rewriting are stored in the EEPROM in addition to address and data of a register which are necessary in optimum adjustment for every liquid-crystal display. Particularly, when the register address value stored in the EEPROM is broken and error determination cannot be made, a case may occur in which unrelated setting is applied to different addresses and display cannot be made. Moreover, because there is a data area which is unnecessary in storing in the EEPROM, an EEPROM section becomes large and it is difficult to decrease a circuit in size and price.
By using an area for storing the backup data of display quality of a display unit, it is possible to automatically change display quality to backup data even at the time of an EEPROM error and keep a display state without through the CPU. Moreover, a technique capable of decreasing a liquid-crystal display in size is desired.
[Patent Document 1] Japanese Patent Laid-Open No. 2004-21067
[Patent Document 2] Japanese Patent Laid-Open No. 2003-241730